The Rotor Dynamic Coefficients of Eccentric Mechanical Face Seals

2017

Mechanical face seals are constitutive components of much larger turbomachines and require consideration of the system dynamics for successful design. The dynamic interplay between the seal and rotor is intensified by recent trends toward reduced clearances, higher speeds, and more flexible rotors. Here, the “rotor” consists of the flexible shaft and the rotating seal seat. The objective here is to, for the first time, determine how the rotor affects the seal performance and vice versa. Thresholds can then be established beyond which the rotor influences the seal but not vice versa (i.e., the rotordynamics can be sent to the seal analysis as an exogenous input). To this end, a model of a flexibly mounted stator face seal is provided including the coupled dynamics of the flexible rotor. The model accounts for axial and angular deflections of the rotor and seal. Coupled rotordynamics are modeled using a lumped-parameter approach including static and dynamic rotor angular misalignments. For expediency, linearized expressions for fluid forces are used, and the resulting steady-state equations of motion are solved analytically to investigate how rotor inertia, speed, and angular misalignment influence the coupled seal dynamics. Importantly, results from the study reveal that in some operating regimes, neglecting the rotordynamics implies healthy seal operation when instead intermittent rub exists between the faces. This work also shows that when the rotor inertia is much larger than the seal inertia, the rotordynamics can be solved separately and used in the seal model as an external input.

Section: Film Thickness Between the Flexibly Mounted Statormentioning

confidence: 99%

“…They restrict their analysis to the case of equal seal face inertia, which is justified since the rotating seal ring is considered to be the rotor. The model is later expanded to include lateral seal ring deflections [15], where constant synchronous shaft whirl couples the seal's angular vibration and the lateral whirl frequency [16].…”

Section: Introductionmentioning

confidence: 99%

Steady-State Response of a Flexibly Mounted Stator Mechanical Face Seal Subject to Dynamic Forcing of a Flexible Rotor

Varney

2017

“…Expressed in the principal system of the element, the normalized support moments for element n are (Wileman, 1994) M^", = -k"{y" -y"i cos i/^J -rf,"t" My", = -d,"il/"y" ~ ks"y"i sin ijj"…”

Section: Dynamic and Applied Momentsmentioning

confidence: 99%

Steady-State Analysis of Mechanical Seals With Two Flexibly Mounted Rotors

Wileman

1997

Self Cite

The dynamic behavior of a mechanical face seal with two flexibly mounted rotors is investigated. The equations of motion are derived using linearized rotor dynamic coefficients to model the dynamic behavior of the fluid film. The equations are shown to be linear in the inertial reference with harmonic forcing functions which result from the initial misalignment of the flexible supports. A method for obtaining the steady-state response in the system is derived by transforming the equations of motion into reference frames which rotate with the shafts. The resulting equations contain constant forcing functions and can be readily solved for the magnitude of the steady-state response. The method presented allows a rapid determination of the steady-state misalignment of a seal without resorting to numerical modeling.

“…Extensive literature reviews have been provided by Allaire (1984), Tournerie and Frene (1985), and Etsion (1982Etsion ( , 1985Etsion ( , and 1991. Wileman and Green (1991, 1996 have described a configuration in which both elements are flexibly mounted to rotating shafts, denoting it FMRR (Fig. 1).…”

Section: Introductionmentioning

confidence: 99%

“…Green and Etsion (1985) determined the stability criteria and steady-state response for the FMS configuration, and Green (1987Green ( , 1989Green ( , and 1990 extended the results to the FMR configuration. Wileman andGreen (1991, 1997) obtained the rotor dynamic coefficients for the FMRR configuration, then derived the equations of motion and developed a method to determine the steady-state response of the system. This work analyzes the equations of motion derived by Wileman and Green (1997) to provide a solution for the stability limits of the FMRR configuration.…”

Section: Introductionmentioning

confidence: 99%

Stability Analysis of Mechanical Seals With Two Flexibly Mounted Rotors

Wileman

1998

Self Cite

Dynamic stability is investigated for a mechanical seal configuration in which both seal elements are flexibly mounted to independently rotating shafts. The analysis is applicable to systems with both counterrotating and corotating shafts. The fluid film effects are modeled using rotor dynamic coefficients, and the equations of motion are presented including the dynamic properties of the flexible support. A closed-form solution for the stability criteria is presented for the simplifled case in which the support damping is neglected. A method is presented for obtaining the stability threshold of the general case, including support damping. This method allows instant determination of the stability threshold for a fully-defined seal design. A parametric study of an example seal is presented to illustrate the method and to examine the effects of various parameters in the seal design upon the stability threshold. The fluid film properties in the example seal are shown to affect stability much more than the support properties. Rotors having the form of short disks are shown to benefit from gyroscopic effects which give them a larger range of stable operating speeds than long rotors. For seals with one long rotor, counterrotating operation is shown to be superior because the increased fluid stiffness transfers restoring moments from the short rotor to the long.